The FERRUM project: Metastable lifetimes in Crii

Abstract

Parity forbidden radiative transitions from metastable levels are observed in spectra of low-density astrophysical plasmas. These lines are used as probes of the physical conditions, made possible due to the long lifetime of their upper level. In a joint effort, the FERRUM project aims to obtain new and accurate atomic data for the iron-group elements, and part of this project concerns forbidden lines. The radiative lifetimes of the metastable energy levels 3 d 4(a 3 D)4 s c 4 D 5/2 and 3 d 4(a 3 D)4 s c 4 D 7/2 of singly ionized chromium have been measured. The experiment has been performed at the ion storage ring CRYRING. We employed a laser-probing technique developed for measuring long lifetimes. In this article, we present the lifetimes of these levels to be τ 5/2= 1.28(16)s and τ 7/2= 1.37(7)s, respectively. A comparison with previous theoretical work shows good agreement and the result is discussed in a theoretical context. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.